<template>
    <lightning-input 
    label="Sold Production" 
    name="num1" 
    onchange={SoldProduction}
    required>
    </lightning-input>

    <lightning-input
      label="Design Production"
      name="num2"
      onchange={DeisgnProduction}
      required>
    </lightning-input>

    <lightning-button label="Calculate Production Differnce" onclick={calcExpr}></lightning-button>

    <lightning-input
      label="Result"
      name="result"
      read-only="true"
      value={result}>   
    </lightning-input>

  </template>

import { LightningElement, api, track } from 'lwc';
import {FlowAttributeChangeEvent,FlowNavigationNextEvent} from 'lightning/flowSupport';

export default class calculator extends LightningElement {
  @track num1;
  @track num2;
  @track result;
  @api result;
  @api num1;
  @api num2;
  @api required;

  calcExpr() {
    this.result = (Number(this.num1) - Number(this.num2)) / ((Number(this.num1) + Number(this.num2))/ 2);
  }

  SoldProduction(evt) {
    this.num1 = evt.target.value;
  }

  DeisgnProduction(evt) {
    this.num2 = evt.target.value;
  }


}

<?xml version="1.0" encoding="UTF-8"?>
<LightningComponentBundle xmlns="http://soap.sforce.com/2006/04/metadata">
  <apiVersion>47.0</apiVersion>
  <isExposed>true</isExposed>
  <masterLabel>Calculator</masterLabel>
  <description>Calculator to find percentage</description>
  <targets>
      <target>lightning__FlowScreen</target>
  </targets>
<targetConfigs>
       <targetConfig targets="lightning__FlowScreen">
        <property name="result" type="Integer" />
        <property name="num1" type="Integer"/>
        <property name="num2" type="Integer"/>
        <property name="required" type="Boolean" default="true"/>
    </targetConfig>
</targetConfigs>
</LightningComponentBundle>

• I have two numbers from a record "num1" and "num2"

• I am not sure how to take these two values and use them as default values when the LWC loads. 

• Once the values change the dafault values should not be used.